By generating over 36 million metric tons (MT) of material and $270 billion of production per year, and being responsible for approximately 3.2 million jobs, plastics and related businesses represent the fourth largest industry in the United States, according to “Plastics in the USA”, Society of Plastics Industry, Washington, D.C. Unlike other material industries, such as steel and aluminum, however, this industry depends almost solely on nonrenewable raw material, mostly imported petroleum. This dependence becomes even more significant as the growth rate of plastics continues to outpace that of all other materials.
Most of the plastic supplied by today's manufacturers ends its life in landfills or incinerators simply because the technology has not been available to recover it economically. The Environmental Protection Agency (EPA) estimates that the amount of plastic in municipal solid waste (MSW) grew from less than 1 million MT before 1960 to over 20 million MT's by 2000.
Durable goods such as automobiles, appliances and electronic equipment account for about one-third of the plastics in MSW. Durable goods are increasingly being collected and partially recycled at the end of their useful lives to avoid disposal costs and potential liabilities, and to recover metals and other marketable raw materials. Take-back and producer-responsibility legislation is also growing.
The recovery of plastics from durable goods requires a plastic-rich raw material. Automobiles, appliances and electronics generally contain more metal than plastic (typically plastic content is less than 30%), so a metal recovery operation must precede plastic recovery. Most metal recovery operations shred equipment in order to cost-effectively liberate metal from whole parts. A large-scale plastic recovery operation must be able to source this plastic-rich raw material from a number of metal recovery operations.
In order to create high value products from this plastic-rich raw material, the plastic recycling process must be able to separate highly mixed streams. The separation must be done on a flake-by-flake basis in order to achieve high throughput rates of products with acceptable purity. (see J. Brandrup, Recycling and Recovery of Plastics, Carl Hanser Verlag, New York, 1996 and D. F. Arola, L. Allen, and M. B. Biddle, “Evaluation of Mechanical Recycling Options for Electronic Equipment”, IEEE Proceedings, May 11-13, 1999, Danvers, Mass.)
One of the significant obstacles to cost effective recovery of the plastic from waste durable goods is processing the plastic-rich raw material from a metal recovery operation such that it can be economically transported to a plastics recovery facility. In addition to plastic-rich raw materials, many waste streams contain significant amounts of metal, fluff, foam, fines, wood and paper that are not economical to ship to plastic recycling facilities since they are not generally target products. Moreover, since the plastics-rich raw material is typically a shredded material, the particle size can be too large to achieve the high bulk density necessary for economical transport. In addition, some of the metal found in the plastic-rich raw material can often be large enough to damage some types of size reduction equipment.
There are many devices available for reducing the size of waste materials including granulators, shredders, grinders, roll mills, knife mills and others. These devices vary in effectiveness based on the material being reduced. Some devices can deal with a significant fraction of metal in the feedstock, but these are general very high horsepower devices.
There are also several techniques for removal of metal from non-metals. These include magnets, metal detectors, eddy current devices and air aspirators.
A number of techniques also exist for the removal of fluff, foam, fines and paper from plastic-rich streams. Such devices include air aspirators, roll sorters, vacuum gravity tables and other devices.